Radiation therapy can be given to treat proliferative tissue disorders including but not limited to cancer, arteriovenous malformations, dermatological lesions etc. During radiation therapy, the tissue of the patient known to or suspected to contain the disease is exposed to radiation. Linear accelerators are commonly used to irradiate a target volume encompassing the tissue to be treated during radiation therapy. As is known, linear accelerators use microwave technology to accelerate electrons in a waveguide and then allow the electrons to collide with a heavy metal target. As a result of the collisions, high-energy x-rays are scattered from the target. A portion of the scattered x-rays is collected and shaped by a beam collimating device to form an output beam of radiation conforming to the shape of the target volume. The linear accelerator also includes a gantry that rotates around the patient allowing the output beam of radiation to be delivered to the desired target volume from any angle by rotating the gantry.
Prior to exposing a patient to radiation, a treatment plan is typically developed in order to determine accurately the location of the tissue to be treated and how best to treat the tissue with radiation. Many imaging techniques have been used in treatment planning such as for example, computed tomography (CT), magnetic resonance imaging (MRI), and nuclear scintigraphy including single photon emission tomography (SPECT) and positron emission tomography (PET). Acquired images of the tissue are used to define the target volume so that the actual tissue irradiated by the output beam of radiation conforms as much as possible to the target volume. In many instances, the images of the tissue used to define the target volume are acquired in a single simulation.
For dose delivery, techniques such as tumour immobilisation with IMRT and image guidance have commonly been utilized. The purpose of image guidance is to ensure that the target tissue is placed at the isocenter of the linear accelerator at the beginning of radiation treatment. In tissue sites where a large amount of tissue motion is expected (for instance lung cancer radiotherapy), image guided therapy also constitutes control of the output beam of radiation to ensure that the irradiation time is restricted to the moment when the tissue is localized at the linear accelerator isocenter.
Unfortunately, this method has a fundamental difficulty if the image used to define the target volume is acquired in a single simulation since it is not known if image guided reproduction of the target location in subsequent treatment fractions results in the planned dosimetry being accurately delivered to the target and non-target tissues. This is because it is not known, a priori, if the single simulation image is representative of the patient positioning and target volume configuration in subsequent radiotherapy treatment fractions.
To provide more accurate position information concerning the target tissue and ensure the beam of radiation is properly directed in subsequent radiotherapy treatment fractions, it has been considered to integrate a linear accelerator with a magnetic resonance imaging apparatus.
MRI is a well-known imaging technique. During MRI, a target, typically a human patient, is placed into an MRI machine and subjected to a uniform magnetic field produced by a polarizing magnet housed within the MRI machine. Radio frequency (RF) pulses, generated by an RF coil housed within the MRI machine in accordance with a particular localization method, are used to scan target tissue of the patient. MRI signals are radiated by excited nuclei in the target tissue in the intervals between consecutive RF pulses and are sensed by the RF coil. During MRI signal sensing, gradient magnetic fields are switched rapidly to alter the uniform magnetic field at localized areas thereby allowing spatial localization of MRI signals radiated by selected slices of the target tissue. The sensed MRI signals are in turn digitized and processed to reconstruct images of the target tissue slices using one of many known techniques.
Integrating a linear accelerator with an MRI apparatus poses several technical problems. For example, the magnetic field generated by the MRI apparatus interferes with the operation of the linear accelerator. In particular, the magnetic field generated within the MRI apparatus interferes with the trajectory of the electron beam in the linear accelerator through the magnetic force F=qvB and can cause the electron beam to deflect. For a strong magnetic field, the deflection can be great enough to force the accelerated electron beam into the accelerating waveguide and prevent it from reaching the heavy metal target at the output of the accelerating waveguide. Even for a partially deflected electron beam, the altered angle of incidence on the heavy metal target may cause sufficient perturbation to the bremstrahlung x-ray beam to cause it to be unacceptable clinically.
In addition, the presence of the linear accelerator perturbs the magnetic field generated by the MRI apparatus. For modern radiotherapy, it is required to move the beam of radiation relative to the patient, in order to conform the radiotherapy to the shape of the target volume. A large amount of material that is placed in the fringe magnetic field of the MRI magnet will cause alteration of the magnetic field lines, which could extend to the homogeneous region of the magnet. This in itself is not a problem since this can be compensated for; however, if this material is moved (for instance if this material were a linear accelerator, or the shielding surrounding a cobalt source), the dynamic perturbation of the magnetic field in the homogeneous region could cause unacceptable image distortions. This problem would exist for both linear accelerator and cobalt based radiotherapy.
Still further problems exist in that the RF fields generated by the linear accelerator interfere with the receiver coils of the MRI apparatus. The linear accelerator works in a pulsed power mode, where microwave frequency RF is generated by pulsing a high voltage current to a microwave generator (a klystron or magnetron), which creates suitable RF power that is transported through a transmission waveguide to the accelerating waveguide. The accelerating waveguide is a periodic structure that generates electric fields that are suitable to accelerate electrons to a Megavoltage energy. The RF fields generated by the linear accelerator are contained in these resonant, transmission and accelerating structures such that no appreciable power will leak out and interfere with the MRI apparatus operation. However, the pulsed power modulator generates high voltage (typically 50 to 100 kV at large currents 70 to 110 A) pulses of typically 4 microsecond duration. The rise and fall times are typically less than 1 microsecond. The frequency spectrum of the pulse contains a component in the MHz range that generates a noise signal of sufficient power that will significantly interfere with the RF receiver coils of the MRI apparatus. The exact frequency and power level of the modulator noise depends on the shape of the modulator high voltage pulse, and the mechanical characteristics of the high voltage circuitry and structure housing the high voltage circuit.
U.S. Pat. No. 6,366,798 to Green discloses a radiotherapy machine including a magnetic resonance imaging system. The radiotherapy machine treats a region of a subject while the region and volumes abutting the region are imaged by the magnetic resonance imaging system. The beam and an excitation coil assembly of the imaging system are arranged so that the beam is not incident on the coil assembly. The excitation coil assembly includes two spaced winding segments for producing a main DC magnetic field. The segments are located on opposite sides of the region. A treatment couch for the subject fits within aligned central openings of the winding segments. The coil assembly produces main magnetic field lines that extend generally in the same direction as the axis about which the beam turns. Mutual interference issues, which arise from placing a rotating beam generator in a stationary magnetic resonance imaging system, are not discussed.
U.K. Patent Document No. 2 393 373 to Lagendijk discloses a linear accelerator integrated with an MRI apparatus. Components and systems are provided that prevent, among other difficulties, the magnetic field of the MRI apparatus to interfere with the operation of the linear accelerator.
U.S. Patent Application Publication No. 2005/0197564 to Dempsey discloses a device and process for performing MR imaging during radiation therapy by using a Helmholtz-pair coil MRI system in conjunction with a cobalt source of radiation. The significant shielding required for the cobalt source may corrupt MR image quality during rotation.
As will be appreciated, there exists a need for an improved integrated linear accelerator and MRI apparatus that obviates or mitigates at least one of the above-identified disadvantages. It is therefore an object of the present invention to provide a novel integrated external beam radiotherapy and magnetic resonance imaging (MRI) apparatus.